In Yu Yu Hakusho (Ghost Fighter), Hiei knew who his sister was immediately without having any second thought. Though he had the Jagan eye, he couldn't see through the barrier that was placed around the place she was confined in the human world. Probably sibling instinct?
When he was still a newborn, he could already understand the people around him but it was never shown in the series that he also saw Yukina as a baby. When he had the Jagan eye, he found the place but not Yukina or Hina. Rui told him few about them but never her appearance specifically. When he opened his hand to get a single drop of snow it showed him memories that Yukina had. What if when he came to his birthplace he touched them and it showed memories of her too.
How did Hiei recognize his sister?